A distinction is drawn between homodyne and heterodyne architectures for mobile radio receivers. While, in the case of heterodyne mobile radio receivers, a received radio-frequency signal is first of all converted to an intermediate frequency in order subsequently to be converted to baseband, homodyne mobile radio receivers convert the radio-frequency signal to baseband in only one conversion step. Homodyne mobile radio receivers such as these are also referred to as zero-IF or as direct conversion (DC) receivers and are used, for example, in the so-called third-generation mobile radio standard, Universal Mobile Telecommunications System, UMTS.
One system-dependent disadvantage of direct conversion is DC voltage offsets which, on the one hand, may be of a static nature, and on the other hand may be of a dynamic nature. The static offsets are caused inter alia by circuitry-dependent offsets in the individual blocks of the receiver chains, for example as a result of large pairing tolerances of the components.
If the received signal is weak, that is to say the described DC offsets may be many times higher than the actual useful signal, the offsets are also amplified with the baseband amplification that is required for the useful signal, so that a signal which is too large for digitization would be produced at the output of the analog baseband chain and at the input of the analog/digital converter that is normally provided there. It is therefore essential to use circuitry measures to compensate for such DC voltage offsets.
Conventional methods to compensate for a DC voltage offset in the analog baseband chain are based either on the high-pass filter principle and are provided by means of simple AC couplings, or feedback loops are provided, with the feedback path having low-pass filter characteristics.
Overall, these methods have the disadvantage that, on the one hand, a low cut-off frequency is required for the high-pass filter in order to avoid excessively distorting the useful signal, while, on the other hand, a high cut-off frequency is required in order to ensure that the stabilization time of the AC coupling is not too long. Furthermore, many direct converters have adaptive gain control which is dependent on the received field strength of the radio-frequency signal. However, gain control systems such as these result in sudden changes in the gain, resulting in transient equalization processes which can exceed the useful signal by many times, so that the analog/digital converter cannot be driven ideally.
The described problems relating to transient equalization processes resulting from changes to gain factors are exacerbated by the fact that such transients would be amplified many times further by subsequent amplifier stages, for example programmable amplifiers.
In the case of mobile radio methods such as GSM, which operate using Time-Division Multiple access TDMA and accordingly transmit and receive in time slots, the described problems can be avoided by making changes to the gain only between time slots. In the case of mobile radio methods for which continuous reception is required, for example in the case of systems which operate using CDMA, Code Division Multiple Access methods, it is however, desirable to match the gain to the received field strength even when the receiver is being operated without any pauses.
The object of the present invention is to specify a receiver arrangement with AC coupling in which it is possible to match the gain in the receiver during a normal reception mode.